public class SingletonDemo4 {  
      
    private static class SingletonClassInstance {  
        private static final SingletonDemo4 instance = new SingletonDemo4();  
    }  
      
    // 方法没有同步，调用效率高  
    public static SingletonDemo4 getInstance() {  
        return SingletonClassInstance.instance;  
    }  
      
    private SingletonDemo4(){}  
}